This application claims the benefit of Korean Patent Application No. 2000-42534, filed on Jul. 24, 2000, which is hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal display devices implenting in-plane switching (IPS) where an electric field to be applied to liquid crystals is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired light image can be produced.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin, and have low power consumption characteristics. The typical liquid crystal display (LCD) panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs), and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
FIG. 1 is a schematic cross-sectional view illustrating a conventional LCD cell in an active matrix LCD. As shown, the LCD cell 20 has lower and upper substrates 2 and 4 and a liquid crystal (LC) layer 10 interposed therebetween. The lower substrate 2 has a thin film transistor (TFT) xe2x80x9cSxe2x80x9d as a switching element that switches a voltage that changes the orientation of the LC molecules. The lower substrate 2 also includes a pixel electrode 14 that is used to apply an electric field across the LC layer 10 in response to signals applied to the TFT xe2x80x9cSxe2x80x9d. The upper substrate 4 has a color filter 8 for producing a color, and a common electrode 12 on the color filter 8. The common electrode 12 serves as an electrode that produces the electric field across the LC layer (with the assistance of the pixel electrode 14). The pixel electrode 14 is arranged over a pixel portion xe2x80x9cP,xe2x80x9d i.e., a display area. Further, to prevent leakage of the LC layer 10, a pair of substrates 2 and 4 are sealed by a sealant 6.
As described above, since the common and pixel electrodes 12 and 14 of the conventional LCD panel are positioned on the upper and lower substrates 4 and 2, respectively, the electric field induced between them is perpendicular to the lower and upper substrates 2 and 4. The described liquid crystal display device has advantages of high transmittance and a high aperture ratio. Furthermore, because the common electrode 12 on the upper substrate 4 acts as a ground, the liquid crystal is shielded from static electricity.
However, the conventional LCD panels having the longitudinal electric field has a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates.
A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to FIGS. 2 and 3A to 3D.
As shown in FIG. 2, lower and upper substrates 30 and 32 are spaced apart from each other, and a liquid crystal 10 is interposed therebetween. The lower and upper substrates 30 and 32 are often referred to as array substrate and color filter substrate, respectively. On the lower substrate 30 are a pixel electrode 34 and a common electrode 36. The pixel and common electrodes 34 and 36 are aligned parallel to each other. On a surface of the upper substrate 32 is a color filter layer 42 that is commonly positioned between the pixel electrode 34 and the common electrode 36 of the lower substrate 30. An overcoat layer 44, which protects the color filter layer 42, is formed on the color filter layer 42. A voltage applied across the pixel and common electrodes 34 and 36 produces an electric field 35 through the liquid crystal xe2x80x9cLC.xe2x80x9d The liquid crystal xe2x80x9cLCxe2x80x9d has a negative dielectric anisotropy, and thus it aligns parallel to the electric field 35. An edge sealant 40 is formed around the edges of the lower and upper substrates 30 and 32, and bonds the upper substrate 32 to the lower substrate 30 to prevent leakage of the liquid crystal xe2x80x9cLCxe2x80x9d.
FIGS. 3A to 3D conceptually help illustrate the operation of a conventional IPS-LCD device. When no electric field is produced by the pixel and common electrodes 34 and 36, i.e., off state, as shown in FIGS. 3A and 3B, the longitudinal axes of the LC molecules xe2x80x9cLCxe2x80x9d are parallel and form a definite angle with the pixel and common electrodes 34 and 36. For example, FIG. 3B shows a common angle of 45 degrees between a line that is perpendicular to the pixel and common electrodes 34 and 36 and the longitudinal axes of the LC molecules.
On the contrary, when an electric field is produced by the pixel and common electrodes 34 and 36, i.e., on state, as shown in FIGS. 3C and 3D, because the pixel and common electrodes 34 and 36 are on the lower substrate 30, an in-plane electric field 35 that is parallel to the surface of the lower substrate 30 is produced. Accordingly, the LC molecules xe2x80x9cLCxe2x80x9d twist to bring their longitudinal axes into coincidence with the electric field. Thus, as shown in FIG. 3D, the LC molecules align with their longitudinal axes parallel with a line perpendicular to the pixel and common electrodes 34 and 36.
In the above-mentioned IPS-LCD panel, there is no common electrode on the color filter. Furthermore, since the above-mentioned IPS-LCD panel has the pixel electrode and the common electrode on the array substrate, it uses the parallel electric field to the array substrate.
Now, referring back to FIG. 2, the overcoat layer 44 is formed on the color filter layer 42 so as to cover and protect the color filter layer 42. Further, the edge sealant 40 is formed around the periphery of the IPS-LCD panel. However, there are some problems in the edge sealant 40 and the overcoat layer 44.
In general, a number of ions are contained in the edge sealant 40. As time passes, these ions migrate into the liquid crystal layer 10 after the LCD panel is complete. In other words, since the edge sealant 40 is formed of a epoxy-based resin that has a great water resistance, the edge sealant 40 includes sodium ions (Na+), chlorine ions (Clxe2x88x92), potassium ions (K+) and/or fluorine ions (Fxe2x88x92), and these ions flow out as time passes. As these ions migrate through the liquid crystal layer 10, they deteriorate the liquid crystal layer 10 and act as defects therein, thereby shortening life of the liquid crystal layer 10.
Furthermore, the color filter layer 42 contains a number of ions, but the overcoat layer 44 prevents these ions from coming out from the color filter 42. However, the overcoat layer 44 also contains a number of ions. The ions in the overcoat layer 44 also migrate into the liquid crystal layer 10 as time passes, thereby accelerating the deterioration of the liquid crystal layer 10. Since the overcoat layer 44 is commonly made of an acryl-based resin, this overcoat layer 44 contains sodium ions (Na+), potassium ions (K+), iron ions (Fe2+/Fe3+), aluminum ions (A13+), etc.
When the liquid crystal layer contains ions as described above, the driving voltage used to create the electric fields in the liquid crystal during operation of the liquid crystal is changed because of the presence of these ions. Accordingly, quality of the liquid crystal panel display degrades as the liquid crystal panel is used over time.
To overcome the display degradation caused by the migration of ions into the liquid crystal, an auxiliary line is used in a non-display area of the array substrate. FIG. 4 is a plan view of an array substrate illustrating a conventional in-plane switching mode liquid crystal display device that has such an auxiliary line. As shown in FIG. 4, the array substrate is divided into a pixel area and a non-pixel area. In the pixel area, a plurality of thin film transistors (TFTs), a plurality of pixel electrodes and a plurality of common electrodes 50 are arranged. On the other hand, a electrostatic discharge device 54 and an auxiliary line 52 are arranged in the non-pixel area. The auxiliary line 52 receives a signal that is applied to the common electrodes 50.
Still, referring to FIG. 4, since the signal applied to the auxiliary line 52 is a direct electrical current with a potential of several volts, an electric field is generated around the auxiliary line 52. Therefore, the ions permeated through the liquid crystal layer from the edge sealant and overcoat layer gather around the auxiliary line 52. However, in this case when the auxiliary line 52 receives the direct electrical current, the auxiliary line 52 catches only the ions that have a polarity opposite to the polarity of that direct electrical current. Namely, since the signal applied to the auxiliary line 52 commonly has a positive polarity, the auxiliary line 52 attracts the negative ions. Accordingly, positive ions still exist in the liquid crystal layer, thereby deteriorating the liquid crystal layer and decreasing the display quality of the liquid crystal panel.
Accordingly, the present invention is directed to an array substrate for an LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an LCD device having a stable image display.
Another object of the present invention is to provide an array substrate for an LCD device, which has a structure that prevents a liquid crystal layer from deteriorated.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to achieve the above object, an embodiment in accordance with the principles of the present invention provides a liquid crystal display device that includes first and second substrates that are divided in a display area and a non-display area; a plurality of switching devices on the first substrate; first and second lines that apply signals to each switching device; a plurality of electrodes on the first substrate; a first auxiliary line arranged in the non-display area, the first auxiliary line receiving a first signal; a second auxiliary line arranged in the non-display area, the second auxiliary line receiving a second signal; and a liquid crystal layer between the first and second substrates.
The switching device includes a thin film transistor. The first line includes a gate line, while the second line includes a data line. The electrodes include a pixel electrode. A plurality of second electrodes includes a common electrode.
The plurality of the second electrodes receives the first signal, while the first line receives the second signal. The first auxiliary line is parallel with the first line, while the second auxiliary line is parallel with the first line.
The first auxiliary line includes a common line. Further, the second auxiliary line receives a signal having opposite polarity to a signal applied to the first line.
The liquid crystal display device further includes a plurality of pad portions in the non-display area and a plurality of electrostatic discharge device in the non-display area.
In another aspect, the principles of the present invention provide an array substrate for a liquid crystal display device, which includes a substrate having a display area and a non-display area; a plurality of switching devices arranged in the display area of the substrate; first and second lines that apply signals to each switching device; a plurality of electrodes on the substrate; a first auxiliary line arranged in the non-display area, the first auxiliary line receiving a first signal; and a second auxiliary line arranged in the non-display area, the second auxiliary line receiving a second signal.
The second auxiliary line receives a same signal as the first auxiliary line. Further, the first line is a gate line.
Furthermore, the second auxiliary line receives a periodically contrary signal from the first auxiliary line.
A plurality of the second electrodes receive the first signal, while the first line receives the second signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.